update summary.md with info on handling of html tags
Add markdownify template filter

Note that this is a Markdownify filter, and is named as such; it's not a Asccidoc filter or in any way connected to a Page.

Fixes #524
Doc for markdownify

This small function feels important enough to maybe deserve more than these three lines, but this will have to do for now.

This assumes that #652 gets merged.
adding appvoyer support for Windows CI integration
Adding Prev/Next functionality to all lists of pages (sections, taxonomies, etc)